Originally my setup was Windows XP Pro x86 with two HDDs. Yesterday I installed my student license of Windows7 x64 on the second HDD. Both OSs were working properly, so I decide to format the Windows XP partition. After I rebooted Windows 7 will not boot anymore. There is a short error message during boot which is "boot0: error".

I'm not an expert on dual booting... not my cup of tea. However with XP initially setup on your first HDD, I am presuming that HDD1 was/is your "boot/system" disk. When you formated that, you wiped some important things such as boot.ini!

Off the top of my head; if you want it back the way it was, get a bootable XP disk and reinstall XP on the primary disk and edit the boot.ini to point to the additional Win7 installation on HDD2.

If you have Data Recovery tools, you try an unformat, but meh...

A third option would be ot install Win7 on the first HDD, and then move over your data from the second disk. You may need to import the second disk before the new Win7 installation on HDD1 will see it. I'd then wipe the second disk!